A 3D reconstruction of a mapped region of an object, in particular of a patient, can then be derived if mappings are recorded from different directions. The 3D reconstruction in turn allows cross-sectional images to be calculated. To record the object from different directions, an x-ray C-arm is conventionally moved at equal angular distances through an angle of 180°. With the positions defined by the angular distances, an x-ray image (a so-called projection) is recorded in each instance.
To ensure that a region of interest of the object can actually be reconstructed, it is necessary for this region to be completely mapped in each of the recorded x-ray images. This is then problematical if the region of interest is located beyond the center point of the object, which usually forms the rotation center. This is the case for instance if the liver of a patient is to be mapped. The region of interest, which lies outside the center point of the patient, can not define a new center point for a circular movement, because otherwise the x-ray source or the x-ray detector would knock against the patient.